Rapid Formation of Copper Patinas: A Simple Chemical Demonstration of Why the Statue of Liberty Is Green

Copper metal is slowly oxidized when it is exposed to the atmosphere for long periods of time, forming a blue-green coating called a patina. For example, the green color of the Statue of Liberty is a result of patina formation. A wide variety of Cu2+ compounds are found in patinas, and they generally take long periods of time to develop. Here, an activity is presented in which patinas are rapidly generated on the surface of copper-containing coins. The process can be carried out using very simple materials, making this activity suitable for a wide range of audiences, and also for courses offered in an online format. Because the development of the patina occurs within tens of seconds, the experiment can be presented as an in-class demonstration. Given the vast number of conditions that can be varied in this activity, it is highly amenable to exploratory learning exercises.